The Tsar and the art collection that came in plain brown envelopes

When the packages in brown paper envelopes arrived, the Russian Court had been firmly instructed that only Tsar Nicholas I himself could open them.

For the special deliveries from his trusted envoys in Paris were filled with 18th century French erotica and, after casting a lusty eye over his latest acquisitions, the Russian monarch would stash them away in his secret library. For more than 200 years, the Tsar's private collection of 40 artworks have lain in storage in the vaults of the State Hermitage Museum in St Petersburg, deemed too explosive to be displayed during the Soviet regime.

But, from today, more than 100 engravings, etchings and objects, including explicit images that blurred the boundaries between art and pornography in the 18th century, can be seen at a new exhibition in Somerset House's Hermitage Rooms, "The Triumph of Eros: Art and Seduction in 18th Century France." The engravings depict anything from a playful Cupid sitting on Venus's lap, to bare-bottomed lovers and masturbating women.

The exhibition - featuring Rococo works by Boucher and etchings by Fragonard as well as an iconic marble sculpture called Menacing Cupid by Etienne-Maurice Falconet, which was commissioned by Madame de Pompadour - was triggered by the recent discovery of the Tsar's library.

Erotically charged aspects of the show feature an image of a woman revelling in the pleasure of her own body while she is overlooked by Cupid in The Shift Withdrawn, by Guersant, as well as two illustrations which were labelled "cabinet paintings", because they were kept locked in cabinets. One, called The Packsaddle shows a woman without underwear and another features a woman whose bare buttocks are being inspected by a man, called The Mare of Peasant Pierre.

Barnaby Wright, curator of the Hermitage Rooms, said the collection also contained a number of unpublished workings by artists, often of body forms without clothing. These were produced by the artist under the guise of conducting a "work in progress" and illicitly collected for their erotic nature.

"You can see from some of the etchings that it is the same image but one has a piece of drapery covering a women and the other has it exposed," said Mr Wright. "The latter was the artist's working copy, which was only legally allowed to be produced 12 times.

"The exposed one was not supposed to be distributed or sold but there was a market among wealthy collectors, and these prints were sold illicitly and there are five such prints, with no lettering at the bottom, owned by Tsar Nicholas, in this exhibition."

The show also hopes to shed insight into changing sexual attitudes during the 18th century. Some images, which deal with the then popular subject of young women contemplating love and sexual desire alone in their boudoirs, can be regarded as the expression of a backlash against Enlightenment rationality, as well as the "disruptive" presence of Cupid. Satish Padiyar, co-curator of the exhibition, said the "cult of Cupid" at this period in France came to demonstrate the anarchic influence that romance, love and sexual desire could exert on an otherwise ordered universe. "You have some wonderful, charming and seductive works of art in the exhibition but there is also a more serious point being made here," he said. "We think of 18th century France representing the age of Enlightenment but the images represent anti-order and passion, and not being in control of things. What disrupts order is the presence of Cupid."

He added that modern-day sexual freedom could be traced back to this period of European cultural history, when paintings were articulating the first expressions of sexual liberation, particularly for women.